Technical Field
The invention relates generally to fuel assemblies for nuclear reactors of the type incorporating a bundle of fuel rods disposed at the nodal points of a regular array and spacer grids distributed along the bundle. The invention relates more particularly to grids for use in fuel assemblies, which include at least two series of mutually parallel plates or straps defining cells which receive fuel rods and possibly elements substituted for them at certain locations in the array.
Although the invention is general in scope, it is particularly suitable for use in fuel assemblies whose rods are arranged in a triangular array whose pitch is "tight", i.e., only a little greater than the diameter of the rods. The use of such a triangular lattice is desirable in an undermoderated reactor core, which means that the coolant streams between the rods must be narrow. However, the construction should allow the required coolant flow and provide adequate mixing of the coolant streams, without causing an excessive head loss.
The grids of nuclear fuel assemblies for water moderated and cooled reactors, and particularly PWRs, generally retain the rods at the nodal points of a square array. The grids typically include a girdle and two sets of orthogonal plates. A fuel assembly has also been proposed U.S. Pat. No. 3,068,163 having grids for maintaining the rods at the nodal points of a square array, which grids are formed by a flat or undulating endless strip passing between the rods, with intermeshing at the crossing points. This solution, if applied to a "tight" pitch fuel assembly, does not allow satisfactory coolant flow, conditions, mixing of the different streams and an acceptable pressure loss. Support grids for retaining a triangular array of fuel rods have also been proposed, for instance in European No. 0 065 613 (Downs), however including a single layer of intersecting corrugated strips arranged along two different directions.